Greenstone belt

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Kilimafedha greenstone belt in Tanzania

Greenstone belts are zones of variably

sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss
bodies.

The name comes from the green hue imparted by the colour of the metamorphic

amphiboles. Greenstone belts also often contain ore deposits of gold, silver, copper, zinc, and lead
.

A greenstone belt is typically several dozens to several thousand kilometres long. Typically, a greenstone belt within the greater volume of otherwise homogeneous

volcanic or sedimentary rocks. As such, understanding the nature and origin of greenstone belts is the most fruitful way of studying Archaean
geological history.

Nature and formation

Metamorphosed pillow lava
from the Ishpeming Greenstone Belt in Michigan

Greenstone belts have been interpreted as having formed at ancient oceanic spreading centers and island arc terranes.

Greenstone belts are primarily formed of volcanic rocks, dominated by basalt, with minor sedimentary rocks inter-leaving the volcanic formations. Through time, the degree of sediment contained within greenstone belts has risen, and the amount of ultramafic rock (either as layered intrusions or as volcanic komatiite) has decreased.

There is also a change in the structure and relationship of greenstone belts to their basements between the Archaean where there is little clear relationship, if any, between basalt-peridotite sheets of a greenstone belt and the granites they abut, and the Proterozoic where greenstone belts sit upon granite-gneiss basements and / or other greenstone belts, and the Phanerozoic where clear examples of island arc volcanism, arc sedimentation and ophiolite sequences become more dominant.

This change in nature is interpreted as a response to the maturity of the plate tectonics processes throughout the Earth's geological history. Archaean plate tectonics did not take place on mature crust and as such the presence of thrust-in allochthonous greenstone belts is expected. By the Proterozoic, magmatism was occurring around cratons and with established sedimentary sources, with little recycling of the crust, allowing preservation of more sediments. By the Phanerozoic, extensive continental cover and lower heat flow from the mantle has seen greater preservation of sediments and greater influence of continental masses.

Greenstones, aside from containing basalts, also give rise to several types of metamorphic rocks which are used synonymously with '

metabasalt' et cetera; greenschist, whiteschist and blueschist
are all terms spawned from the study of greenstone belts.

The West African early Proterozoic greenstone belts are similar to the Archean greenstone belts. These similarities include a decrease in the amount of

Calc-alkaline dikes are common in these suites.[1]

Distribution

Weathered Precambrian pillow lava in the Temagami greenstone belt on the Canadian Shield

Archaean greenstones are found in the

Kaapvaal craton and also in the cratonic core of Madagascar, as well as West Africa and Brazil, northern Scandinavia and the Kola Peninsula (see Baltic Shield
).

Proterozoic greenstones occur sandwiched between the Pilbara and Yilgarn cratons in Australia, and adjoining the Gawler craton and within the extensive Proterozoic mobile belts of Australia, within West Africa, throughout the metamorphic complexes surrounding the Archaean core of Madagascar; the eastern United States, northern Canada and northern Scandinavia. The Abitibi greenstone belt in Ontario and Quebec is one of the largest Archean greenstone belts in the world.

In Antarctica, the Proterozoic-aged

Fisher Massif closely resembles the composition and structure of a greenstone belt.[2]

One of the best known greenstone belts in the world is the South African

spinifex textures created by crystals
formed under rapidly cooling environments, namely water.

List of greenstone belts

Main article: List of greenstone belts

Africa

Asia

Europe

North America

South America

Australia

References

  1. ^ Huot, Daniel; Sattran, Vladimir (1987). "Gold in Birrimian greenstone belts of Burkina Faso, West Africa".
    doi:10.2113/gsecongeo.82.8.2033
    .
  2. ^ "Prince Charles Mountains". Australian Antarctic Division. 30 July 2002. Archived from the original on 2 April 2015. Retrieved 14 November 2019.
  3. ^ Manikyamba, C.; Kerrich, R.; Naqvi, S.M.; Ram Mohan, M. (2004). "Geochemical systematics of tholeiitic basalts from the 2.7 Ga Ramagiri-Hungund composite greenstone belt, Dharwar craton".
    doi:10.1016/j.precamres.2004.05.010
    .
  4. ^ Robertson, James M.; Moench, R.H. (17 October 2023) [c. 2014]. "The Pecos greenstone belt – a Proterozoic volcano-sedimentary sequence in the southern Sangre de Cristo Mountains, New Mexico". nmgs.nmt.edu. Guidebook (revised ed.). Socorro, NM: New Mexico Geological Society. ID 6172. Retrieved 1 March 2017.
  5. ^ Lemen, D.; Lindline, J.; Bosbyshell, H. (2015). The Gallinas Canyon gneiss: A window into the nature and timing of Paleoproterozoic events in northern New Mexico (PDF). Field Conference. Vol. 66. Socorro, NM: New Mexico Geological Society. pp. 185–192. Retrieved 15 June 2020.
  6. ^ Henderson, J.F.; Brown, I.C. (1966). Geology and structure of the Yellowknife Greenstone Belt, District of Mackenzie (zip) (Report). Geological Survey of Canada Bulletin. Vol. 141. p. 38.
    doi:10.4095/101459
    . (lat,lon) = (+49.5°, −114°) to (+49.0°, −113°).
  7. ^ Naipal, R.; Kroonenberg, S.B. (December 2016) [4 May 2016]. "Provenance signals in metaturbidites of the Paleoproterozoic greenstone belt of the Guiana shield in Suriname".
    doi:10.1017/njg.2016.9
    . Available on CJO 2016.
  8. ^ Norseman-Wiluna greenstone belt (PDF). gladiatorresources.com.au (map). Leederville, WA, Australia: Gladiator Resources Ltd. Archived from the original (PDF) on 14 October 2009. Retrieved 6 September 2009.

Sources

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